This invention relates in general to a resonator for damping pressure waves or instabilities supported by acoustic energy within a system and, more specifically, to the use of counter-bored holes to increase the acoustic conductance of a resonator within a system having a combustor. The resonator may be placed at or near the location within the system having the highest acoustic pressure amplitude. This invention also relates to a method of placing a resonator adopting counter-bored holes at or near the locations of the highest acoustic pressure amplitudes within a system to suppress combustion instabilities so that the mass flow of air bypassing the system's combustor is maintained within design tolerances.
One common type of device used for damping the effects of acoustic energy is a Helmholtz resonator. A Helmholtz resonator provides a closed or blind-cavity having one sidewall with openings therethrough. The fluid inertia of the gases within the pattern of holes acts against the volumetric stiffness of the closed cavity, producing a resonance in the velocity of flow through the holes. This flow oscillation has a well-defined natural frequency and provides an effective mechanism for absorbing acoustic energy. Groups of Helmholtz resonators have been used in various types of systems to dampen pressure waves, or instabilities supported by pressure waves, that are generated within the system during operation. Examples of such systems include rockets, ramjets and gas turbine engines. More specifically, Helmholtz resonators have been used in gas turbine combustors to minimize combustion instabilities.
Pertaining to gas turbine combustors, to reduce the amount of NOx emissions from a combustion turbine power plant, it is known to provide a lean-premix fuel to the power plant's combustor. A fuel-lean premix includes a fuel premixed with a large excess of air. While the fuel-lean premix reduces the amount of NOx emissions, high frequency combustion instabilities, commonly referred to as “high frequency dynamics” or “screech oscillations,” can result from burning rate fluctuations inside the combustors, which consume the fuel-lean premix. These burning rate fluctuation instabilities may create damaging pressure waves. It is desirable to provide a means of acoustic damping to suppress the acoustic energy.
Some prior art Helmholtz resonators have been in the form of monolithic liners extending over large areas of the combustion chamber walls. Such monolithic liners can be subject to high thermal stress due to the large temperature differences that may occur between the combustion chamber liner and outer walls of the combustion chamber. Monolithic liners may also be difficult to install as the components may interfere with other components of the combustion turbine power plant. Because of these conflicts, monolithic liners are typically restricted to use near the head of a combustor, which may not be the optimum location for suppressing acoustic energy. Additionally, monolithic liners are typically supported by circumferentially oriented ribs, honeycomb cells, or other means, which provide a compartmentation of the area behind the liner. These structures result in complex, sealed compartment vessel configurations that can be costly to fabricate.
Other types of resonators, such as through-flow resonators, have been placed on support plates upstream of the combustor assembly, see e.g., U.S. Pat. No. 5,373,695. Such resonators, however, are frequently placed in the available space on a cover plate, not necessarily at the location of the highest acoustical pressure amplitude. Through-flow resonators are variants on the traditional Helmholtz blind-cavity resonator. A blind-cavity resonator features a single pattern of holes enclosed on one side by a sealed enclosure. A through-flow resonator includes a second pattern of holes added to the enclosure, which is disposed on the upstream side of the first pattern of holes. This configuration admits a steady purging or scavenging flow of air through the resonator. The flow of air through the resonator permits the frequency-response bandwidth of the resonator to be broadened so that the accuracy and repeatability of frequency tuning are rendered much less critical than in the case of the blind-cavity resonator, which produces a much sharper resonance. The improved accommodation of imperfect tuning improves the likelihood of obtaining good damping performance in the face of uncertainties in temperature and other parameters. Obtaining a more blunt response characteristic improves damping performance. However, due to the length of the flow outlet holes it is difficult to obtain good damping performance when the flow outlet holes must be located in a relatively thick-walled portion of a combustor or transition section, such as those being in excess of 0.10 inch in thickness, while ensuring that the mass flow of air through the resonator stays within allowable limits.
Consequently, there is a need for a through-flow resonator that when situated within a system, such as in a thick-walled portion of a combustor, at or near the points within the system's flow path having the highest acoustical pressure amplitudes will more effectively dampen instabilities without the need for increases in the mass flow of air through the resonator, which would bypass the combustion process.